<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 400px;
            height: 400px;
            background-color: yellow;
        }
        .box::before{
            content:"" ;
            width: 100px;
            height: 100px;
            display: inline-block;
            background-color: aqua;

        }

    </style>
</head>
<body>
    <div class="box"></div>
    
</body>
</html>